The present invention relates to an in-line type color cathode ray tube and, more particularly, to an improved in-line type color cathode ray tube having deflection yokes for generating an improved magnetic field distribution within a deflection plane.
A vacuum envelope in a conventional color cathode ray tube includes a neck received in an electron gun assembly of in-line type, a panel having a faceplate on which a screen, such as a phosphor screen is formed, and a funnel for coupling the neck to the faceplate.
The electron gun assembly includes three electron guns which are horizontally arranged. Electron beams emitted from these electron guns land on a phosphor screen which has red, green, and blue phosphor stripe layers and formed on the faceplate. Thus, a light ray is emitted from these phosphor layers. In this case, the electron beams are selectively landed on predetermined phosphor stripe layers to emit light with adequate color reproduction. For this purpose, a shadow mask with a large number of apertures is arranged in the panel and opposes the faceplate.
The in-line type electron guns are designed such that three electron beams emitted from cathodes thereof always pass through a common plane and are converted near a screen. U.S. Pat. No. 2,957,106 describes a three-electron beam converging technique wherein the center beam and side beams inclined in respect to the center beam, are emitted from the corresponding cathodes. U.S. Pat. No. 3,772,554 describes an electron beam converging technique wherein side beam apertures of some of the electrodes constituting an electron lens are deviated outward in comparison with the side beam apertures of the other electrodes. These conventional techniques have been in popular use for conventional color cathode ray tubes.
An electron beam deflection device is mounted on the outer surface of a conical portion of the funnel to scan the phosphor screen with the electron beams from the electron guns. The electron beam deflection device has a horizontal deflection coil for generating a horizontal deflection magnetic field to horizontally deflect the electron beams, and a vertical deflection coil for generating a vertical deflection magnetic field to vertically deflect the electron beams. In an actual color cathode ray tube, convergence errors may occur when three beams are deflected. In order to eliminate these errors, a technical countermeasure is provided in conventional color cathode ray tubes. This countermeasure is the use of a so-called convergence-free system. According to the conventional technique, the horizontal deflection magnetic field is of a pincushion type, and the vertical deflection magnetic field is of a barrel type. With this countermeasure, the three electron beams are focused on the entire surface of the phosphor screen. Therefore, in the color cathode ray tube employing a convergence-free system, a parabolic current generator for convergence correction and a convergence yoke for generating a convergence correction magnetic field can be omitted. The color cathode ray tube can thus be fabricated at low cost and its productivity can be improved.
Various techniques are adopted in color cathode ray tubes to improve image quality, as described above. However, an increase in screen size of cathode ray tubes poses new problems.
Electron beams respectively emitted from three electron guns are landed on the phosphor screen to form an electron beam spot. An electron beam spot consisting of only a circular core is formed at the central portion of the screen receiving electron beams without deflection. However, an electron beam spot consisting of a flat core and upper and lower flares is formed at a screen corner on which deflected electron beams are directed. As a result, the size of the electron beam spot at the screen corner is increased to degrade focusing and hence, resolution.